Leading wheel

The leading wheel or leading axle or pilot wheel of a steam locomotive is an unpowered wheel or axle located in front of the driving wheels. The axle or axles of the leading wheels are normally located in a truck or "bogie." Leading wheels are used to help the locomotive negotiate curves and to support the front portion of the boiler.

Importantly, the leading bogie does not have simple rotational motion about a vertical pivot, as might first be thought. It must also be free to slip sideways to a small extent (otherwise the locomotive is unable to follow curves accurately – a point lost on the 19th century railway pioneers),[citation needed] and some kind of springing mechanism is normally included to control this movement and give a tendency to return to centre. The sliding bogie of this type was patented by William Adams in 1865.[1] The first use of leading wheels is commonly attributed to John B. Jervis who employed them in his 1832 design for a locomotive with four leading wheels and two driving wheels (a type that became known as the Jervis). In the Whyte system of describing locomotive wheel arrangements, his locomotive would be classified as a 4-2-0: that is to say, it had four leading wheels, two driving wheels, and no trailing wheels. In the UIC classification system, which counts axles rather than wheels and uses letters to denote powered axles, the Jervis would be classified 2A.